In a known membrane fuel pump of the kind referred to above, a major part of the fuel volume pumped in idle flows back via the bypass during the idle mode of a two-stroke engine because the pressure controller allows only a small quantity of fuel to pass into the control chamber. The average pressure measured during idle on the pressure end of the pump is less by a factor of 10 than for a pump without a bypass so that the pressure controller operates without disturbance in accordance with its characteristic and main nozzle drip is substantially avoided.
A significant fuel quantity flows to the pressure controller during full load and only a small quantity of fuel flows off via the throttle of the bypass. In this way, a pumping volume of the pump adapted approximately to the engine speed is obtained at an average pressure to be controlled by the pressure controller. However, it is precisely in the full-load case that the reduction of the pumping capacity is disadvantageous which is caused by the bypass so that the pump must, in most cases, be configured larger to compensate for this reduction.
In addition, operating conditions can occur in the case of idle wherein the pressure drop caused by the bypass, which is in principle wanted, is disturbing. If a higher suction capacity of the pump is required, for example, because of a blocked tank vent, then the drop in pressure provided at the pressure end is disadvantageous. Because of the bypass, such a pressure drop can occur even forward of the pressure controller so that the idle mixture becomes lean and the engine stalls. If the above-described condition occurs during part load, the leaning of the fuel mixture not only causes a disturbing drop in power but a longer duration of operation with a lean mixture can lead to serious engine damage such as seizing of the piston or the like.